1. Field of the Invention
The present invention relates in general to voltage measurement, and, more particularly, to an apparatus and method for measuring a breakdown voltage between two electrodes separated by a dielectric, such as the breakdown voltage, or spark voltage, of a sparkplug or the like.
2. Background Art
Internal combustion engines which include sparkplug ignition systems have been in widespread use for quite some time. The ignition systems energize sparkplugs with voltages sufficient to cause a breakdown, or spark, between two electrodes of the sparkplug. This, in turn, ignites an air-fuel mixture within a cylinder of the engine.
It is often desirable to monitor the performance of an engine. One way of monitoring engine performance is to measure the breakdown voltage of associated sparkplugs, i.e., the voltage at which sparks occur. By monitoring breakdown voltages, sparkplug wear may be monitored. In addition, worn or damaged plug wires may be detected, as well as improperly adjusted air gaps between sparkplug electrodes.
Prior art systems for measuring breakdown voltage typically employ a "nonintrusive" sensor, such as a capacitive sensor, proximate to a plug wire connected to one electrode of the sparkplug. One example of a prior art capacitive sensor is Willenbecher, Jr., U.S. Pat. No. 4,090,130. The plug wire typically couples the electrode to an ignition coil within the ignition system. The output of the sensor is transferred to a peak-hold circuit, which, in turn, outputs a voltage indicative of a peak output of the sensor. An example of a prior art system including a capacitive sensor coupled to a peak-hold circuit is Bayba, U.S. Pat. No. 4,825,167.
Often, an additional spark, or restrike, will occur shortly after sparkplug breakdown. Such restrikes are generally caused by residual energy remaining in the ignition coil subsequent to the original spark.
One shortcoming of prior art voltage measurement systems is that they allow sensor output corresponding to restrikes to be transferred to the peak-hold circuit. This may result in an erroneously high voltage measurement, by allowing the output of the sensor corresponding to a restrike to increase charge held within a storage capacitor of the peak-hold circuit.
The present invention overcomes the shortcoming of such prior art systems by providing restrike clamping means for precluding further transfers from a sensor means to a peak-hold means for a predetermined period of time subsequent to the detected occurrence of a spark.
This and other features of the present invention will become apparent in light of the present specification, drawings and claims.